246 



ZOOLOGY. 



showing the lung and heart (7i) and the mouth (w) as well 

 as the four tentacles, with an eye at the end of the two 

 upper tentacles. Fig. 198 shows the 

 brain and pedal ganglia oi Helix albola- 

 bris. The tentacles when carefully exam- 

 ined may be found to contain both the 

 eyes (e) with the optic nerve (op) and the 

 olfactory nerve (Fig. 301, o). Fig. 199 

 represents the jaw and lingual ribbon of 

 Helix. 



The eggs of the pond-snails are laid in 

 transparent capsules attached to sub- 

 merged leaves, etc. Those of Physa 

 hfterostropTia are laid in the early spring, 

 and three or four weeks later from fifty to 

 sixty embryos with well-formed shells may 

 Fig iu.-Lim.nceysap- be fouud in the capsule. 

 presms.— er ore . rpj^g g^gg ^j LlmncBus are laid late in 



the spring in capsules containing one or two eggs, and sur- 

 rounded by a mass of jelly. After passing through the mo- 



Fig. 195. — Limnc&ue elodes, a common pond-snail, showing its variations. — 

 After IVTorse. 



rula, gastrula, and trochosphere stages a definite veligei 

 stage is finally attained. The foot is large and bilobed, tht 

 mantle and shell then arise, and the definite moUuscan char- 

 acters are assumed, the shell, creeping foot, mantle-flap, eyes, 

 and tentacles appearing, and the snail hatching in about 

 twenty days after development begins. 



Land-snails and slugs lay their eggs loose under damp' 

 leaves and stones, and development is direct, the young 

 snail hatching in the form of the adult. 



